In recent years, attention has been focused on MIMO (Multiple-Input Multiple-Output) as an art for realizing high-speed large-capacity communications in a radio communication technology. The MIMO is an art of transmitting data using a plurality of antennas in both transmission and reception. Different data are transmitted from a plurality of transmission antennas respectively, whereby the transmission capacity can be enhanced without enlarging the time and frequency resources.
In the MIMO, when data are transmitted from a plurality of antennas, a beam transmission method of forming a beam by transmitting weighted data from each antenna is available. The beam transmission has the advantage that the reception power of a terminal is increased due to the beam gain.
Spatial multiplexing using a plurality of beams is also possible, in which case beam transmission suited for the state of a propagation path is performed, whereby the transmission capacity can be improved as compared with spatial multiplexing using antennas. In this case, it is necessary to send information of a beam suited for the propagation path state of the receiving party to the transmitting party.
At present, in 3GPP (3rd Generation Partnership Project) of an international standardization organization of mobile telephones, standardization activity of an LTE (Long Term Evolution) system is carried out as a system for realizing higher-speed larger-capacity communications than communications of the current third generation mobile telephone. Also in the LTE, the MIMO is positioned as an indispensable technology to meet the required condition for high-speed large-capacity transmission. In the LTE, the transmission beam technology is discussed as a technology of pre-coding.
FIG. 11 schematically shows transmission beam formation by pre-coding. For example, to use it for a cellular system for mobile communications of mobile telephones, etc., when a signal is transmitted through multiple antennas from a radio base station 101, the beam number indicating the optimum beam pattern in a user terminal 102 is set according to a feedback signal from the user terminal 102 of a mobile station. The radio base station 101 weights the transmission signal of each antenna in response to the beam number, thereby forming a transmission beam 103 which becomes a beam pattern in an appropriate direction for the receiving user terminal 103.
According to the pre-coding, a beam responsive to the feedback signal from the user terminal is selected for use among the stipulated beam patterns transmitted from the radio base station, whereby the signal strength when a signal is transmitted to the user terminal can be ensured, and in addition, it is made possible to transmit a plurality of signals at the same time using orthogonal beams. Thus, the advantages of coverage enlargement because of improvement in the signal strength and improvement in throughput because of space division can be expected. Such pre-coding is disclosed in non-patent document 1, for example.
In the pre-coding, to reflect the observation state (propagation path state) of a reception signal at the reception point, a beam is controlled using a feedback signal containing beam information transmitted from the receiving user terminal to the transmitting radio base station. In such a case, a feedback signal determination error may occur in the transmitting radio base station because of the factor of fading variation, etc., and in such a case, a beam is transmitted in an erroneous direction as a result of the determination error. Thus, if a determination error occurs in the feedback signal, an appropriate beam cannot be selected and thus the quality at the reception time in the user terminal cannot be ensured. Thus, it is feared that the transmitted data cannot be demodulated and throughput decrease may result. Particularly, for example, if the characteristic of the communication line between the transmitting radio base station and the receiving user terminal is asymmetric and sufficient reliability of the upload propagation path cannot be ensured because of fading, etc., as an error correction code is not contained in the upload communication line of feedback from the user terminal to the radio base station, etc., a determination error may occur in the radio base station.
Patent document 1: 3GPP TSG RAN WG1 #42, R1-050912, Qualcomm Europe, “MIMO proposal for MIMO-WCDMA evaluation”, Aug. 29-Sep. 2, 2005